1. Field of the Invention
The present invention relates generally to transposable elements, and more particularly to the transposon piggyBac.
2. Related Art
Transposable elements (transposons) can move around a genome of a cell and are useful for inserting genes for the production of transgenic organisms. The Lepidopteran transposon piggyBac is capable of moving within the genomes of a wide variety of species, and is gaining prominence as a useful gene transduction vector. The transposon structure includes a complex repeat configuration consisting of an internal repeat (IR), a spacer, and a terminal repeat (TR) at both ends, and a single open reading frame encoding a transposase.
The Lepidopteran transposable element piggyBac was originally isolated from the TN-368 Trichoplusia ni cell culture as a gene disrupting insertion within spontaneous baculovirus plaque morphology mutants. PiggyBac is a 2475 bp short inverted repeat element that has an asymmetric terminal repeat structure with a 3-bp spacer between the 5′ 13-bp TR (terminal repeat) and the 19-bp IR (internal repeat), and a 31-bp spacer between the 3′ TR and IR. The single 2.1 kb open reading frame encodes a functional transposase (Cary et al., 1989; Fraser et al., 1983, 1995; Elick et al., 1996a; Lobo et al., 1999; Handler et al., 1998).
PiggyBac transposes via a unique cut-and-paste mechanism, inserting exclusively at 5′ TTAA 3′ target sites that are duplicated upon insertion, and excising precisely, leaving no footprint (Elick et al., 1996b; Fraser et al., 1996; Wang and Fraser 1993).
Transient excision and interplasmid transposition assays have verified movement of this element in the SF21 AE Spodoptera frugiperda cell line, and embryos of the Lepidopteran Pectinophora glossypiella, Bombyx mori, and T. ni, as well as the Dipteran species Drosophila melanogaster, Aedes aegypti, Aedes triseriatus, Aedes albopictus, Anopheles stephensi and Anopheles gambiae. There is also evidence of transposition in the Cos-7 primate cell line, and embryos of the zebra fish, Danio rerio (Fraser et al., 1995; Buck et al., 1996b; Fraser et al., 1996; Elick et al., 1997; Thibault et al., 1999; Tamura et al., 2000; Lobo et al., 1999).
The piggyBac element has been used successfully as a helper-dependent gene transfer vector in a wide variety of insect species, including the Mediterranean fruit fly, C. capitata, D. melanogaster, Bombyx mori, P. glossypiella, Tribollium casteneum, and Ae. aegypti (Handler et al., 1998, 1999; Tamura et al., 2000; Berghammer et al., 1999).
Excision assays using both wildtype and mutagenized piggyBac terminal sequences demonstrated that the element does not discriminate between proximal or distal duplicated ends, and suggest that the transposase does not first recognize an internal binding site and then scan towards the ends. In addition, mutagenesis of the terminal trinucleotides or the terminal-proximate three bases of the TTAA target sequence eliminates excision at the altered terminus (Elick et al., 1996b).
Although the reported piggyBac vector is useful, length of genes that could be transferred is limited by the size of the other components of the vector. Minimizing the length of the vector to allow more room for the genetic material to be transferred would improve the versatility of the system and reduce costs of preparing synthetic vectors. Previously, the gene to be expressed or transduced was inserted into the middle of the piggyBac transposon in the plasmid p3E1.2. The final construct included the entire length of the piggyBac transposon (2475 bases) and flanking sequences derived from the baculovirus 25K gene region of approximately 813 bases, as well as the plasmid pUC backbone of 2686 bp, and an overall size of approximately 5962 bp. (In cloning sequences into the pUC vector, 12 bp of multiple cloning site DNA was lost). This size limited the effective size of genes that may be inserted, because plasmids larger than 10 KB are generally more difficult to construct, maintain, and transduce into host genomes.
Another problem was that previous cloning regimens involved the excision of a gene, the promoter controlling the gene, and polyadenylation signals, from one plasmid followed by insertion into the piggyBac transfer vector. This procedure was often complicated by the lack of suitable restriction enzyme sites for these manipulations.